right-click the model's animation set and click Add Override Materials. Right-click it again, go to Show in Element Viewer > Model. There should be an array called materials at the bottom, expand it. Now, right-click either briefcase (if you want to modify the RED intel) or briefcase_blue (for the BLU one). Select Add Attribute > float. Call the attribute $detailblendfactor. When the new attribute is created, it defaults to 0, which automatically turns the lights off. You can also make them dimmer by setting it to 0.2 or something like that.

That doesn't work with the HL2 keypad though, I'll see if I can find a way to turn the screen off in the Element Viewer.

Okay, got the HL2 keypad working. Just use the same process, but instead of adding an attribute of type "float" called $detailblendfactor, add a "color" called $selfillumtint. You can then control the lightness of the screen by seting the colour to different shades of grey (or even change its colour entirely, cool stuff). The materials are keypad_sheet (blank screen), keypad_granted (*ACCESS*) and keypad_denied (*ERROR*).